Glass article and method for producing the same

ABSTRACT

A method for producing a glass article is provided. The method for producing a glass article, the method including preparing a glass to be processed, the glass comprising a glass bulk and a low-refractive surface layer disposed on the glass bulk, and etching away the low-refractive surface layer to form an etched glass, wherein the etching away the low-refractive surface layer comprises: cleaning the low-refractive surface layer with an acid solution; and cleaning the low-refractive surface layer with a base solution after the cleaning it with the acid solution.

This application claims priority from Korean Patent Application No.10-2018-0016742 filed on Feb. 12, 2018 in the Korean IntellectualProperty Office, the disclosure of which is incorporated herein byreference in its entirety.

BACKGROUND 1. Field of the Disclosure

The present disclosure relates to a glass article and a method forproducing the same.

2. Description of the Related Art

Glass articles are widely used in electronic devices or constructionmaterials including display devices. For example, glass articles areemployed as a substrate for a flat display device such as aliquid-crystal display (LCD), an organic light-emitting display (OLED)and an electrophoretic display (EPD), or a window for protecting it.

As portable electronic devices such as smart phones and tablet PCsprevail, a glass article employed thereby is frequently exposed toexternal impact. Accordingly, what is required is a glass article thatis thin and thus easy to carry and has good strength for withstandingexternal impact.

SUMMARY

Aspects of the present disclosure provide a method for producing a glassarticle having a good strength.

Aspects of the present disclosure also provide a glass article having agood strength.

It should be noted that objects of the present disclosure are notlimited to the above-mentioned object; and other objects of the presentinvention will be apparent to those skilled in the art from thefollowing descriptions.

According to an aspect of the present disclosure, there is provided amethod for producing a glass article, the method including preparing aglass to be processed, the glass comprising a glass bulk and alow-refractive surface layer disposed on the glass bulk, and etchingaway the low-refractive surface layer to form an etched glass, whereinthe etching away the low-refractive surface layer comprises: cleaningthe low-refractive surface layer with an acid solution; and cleaning thelow-refractive surface layer with a base solution after the cleaning itwith the acid solution.

According to another aspect of the present disclosure, there is provideda method for producing a glass article including preparing a glass to beprocessed, the glass comprising a glass bulk and a low-refractivesurface layer disposed on the glass bulk, and polishing a surface of theglass to form a polished glass, wherein the polishing the surface of theglass comprises: removing at least partially the low-refractive surfacelayer.

According to another aspect of the present disclosure, there is provideda method for producing a glass article including preparing a glass to beprocessed that has a first surface and a second surface opposed to thefirst surface, wherein the glass has a first maximum compressive stressat the first surface and a second maximum compressive stress at thesecond surface, and polishing the first surface and/or the secondsurface to reduce deviations between the first maximum compressivestress and the second maximum compressive stress.

According to an aspect of the present disclosure, there is provided aglass article comprising a glass comprising glass bulk and alow-refractive surface layer disposed on the glass bulk, the glasscomprising a compressive region disposed adjacent to a surface of theglass and a tensile region disposed inside the glass, wherein arefractive index of the low-refractive surface layer is smaller than arefractive index of the glass bulk and is greater than a refractiveindex of air, wherein the low-refractive surface layer is disposedwithin the compressive region, and wherein a thickness of thelow-refractive surface layer is less than 100 nm and is smaller than acompression depth of the compressive region.

According to another aspect of the present disclosure, there is providea glass article comprising a glass comprising a first surface, a secondsurface opposed to the first surface and side surfaces, wherein theglass comprises a first compressive region having a first compressiondepth from the first surface, a second compressive region having asecond compression depth from the second surface, and a tensile regiondisposed between the first compressive region and the second compressiveregion, wherein the glass comprises glass bulk and a side low-refractivesurface layer disposed on side surfaces of the glass bulk, and wherein arefractive index of the side low-refractive surface layer is smallerthan a refractive index of the glass bulk and greater than a refractiveindex of air.

The details of one or more embodiments of the subject matter describedin this specification are set forth in the accompanying drawings and thedescription below.

According to an exemplary embodiment of the present disclosure, a glassarticle can have a high strength which is not easily broken by anexternal impact. According to an exemplary embodiment of the presentdisclosure, a glass article having a high strength can be produced by aneasy method.

It should be noted that effects of the present disclosure are notlimited to those described above and other effects of the presentdisclosure will be apparent to those skilled in the art from thefollowing descriptions.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects and features of the present disclosure willbecome more apparent by describing in detail exemplary embodimentsthereof with reference to the attached drawings, in which:

FIG. 1 is a perspective view of glass articles according to variousexemplary embodiments;

FIG. 2 is a cross-sectional view of a glass article having the shape ofa flat plate according to an exemplary embodiment of the presentdisclosure;

FIG. 3 is a graph showing the stress profile of a glass articleaccording to an exemplary embodiment of the present disclosure;

FIG. 4 is a schematic diagram illustrating an ion exchange processaccording to an exemplary embodiment of the present disclosure;

FIG. 5 is a flowchart for illustrating a method for producing a glassarticle according to an exemplary embodiment of the present disclosure;

FIG. 6 is a cross-sectional view of a strengthened glass including a lowrefractive surface layer;

FIG. 7 is a cross-sectional view showing an acid cleaning process of thestrengthened glass;

FIG. 8 is a cross-sectional view showing a base cleaning process ofstrengthened glass;

FIG. 9 is a graph showing the stress profile before and after theetching process;

FIGS. 10 and 11 are images when viewed from the top for comparing thesurfaces of the glass articles according to the etching techniques;

FIGS. 12 and 13 are graphs illustrating the stress profiles of the glassafter being subjected to the second ion exchange process according tovarious exemplary embodiments;

FIG. 14 is a flowchart for illustrating a method for producing a glassarticle according to another exemplary embodiment of the presentdisclosure;

FIG. 15 is a cross-sectional view showing a polishing process of thestrengthened glass;

FIG. 16 is an image showing a cross section of the glass article afterthe polishing process has been completed;

FIG. 17 is an image of glass article sample 1 when viewed from the top;

FIG. 18 is an image of glass article sample 2 when viewed from the top;

FIGS. 19 to 21 are graphs showing the results of the ball drop tests onthe glass articles according to various exemplary embodiments;

FIG. 22 is a graph showing the compressive stresses of glass articlesamples before and after the etching process;

FIG. 23 is a graph showing the compression depth of the glass articlesamples before and after the etching process;

FIG. 24 is a graph showing the compressive stresses of glass articlesamples before and after the polishing process; and

FIG. 25 is a graph showing the compressive depth of glass articlesamples before and after the polishing process.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Advantages and features of the present disclosure and methods ofaccomplishing the same may be understood more readily by reference tothe following detailed description of exemplary embodiments and theaccompanying drawings. The present disclosure may, however, be embodiedin many different forms and should not be construed as being limited tothe exemplary embodiments set forth herein. Rather, these exemplaryembodiments are provided so that this disclosure will be thorough andcomplete and will fully convey the concept of the present disclosure tothose skilled in the art, and the present disclosure will only bedefined within the scope of the appended claims.

It will be understood that when an element or layer is referred to asbeing “on”, “connected to” or “coupled to” another element or layer, itcan be directly on, connected or coupled to the other element or layeror intervening elements or layers may be present. In the drawings, likereference numerals indicate like elements. As used herein, the term“and/or” includes any and all combinations of one or more of theassociated listed items.

As used herein, “glass article” refers to an article that is entirely orpartially made of glass.

Exemplary embodiments of the present disclosure will hereinafter bedescribed with reference to the accompanying drawings.

The glass is used as a window for protecting a display, a substrate fora display panel, a substrate for a touch panel, an optical member suchas a light guide plate, etc. in electronic devices including a display,such as a tablet PC, a notebook PC, a smart phone, an electronic book, atelevision and a PC monitor as well as a refrigerator and a cleaningmachine including a display screen. Glass may also be employed as acover glass for an instrument panel in a vehicle, a cover glass forsolar cells, interior materials for construction materials, windows forbuildings and houses, etc.

Some glass articles are required to have high strength. For example,when glass is employed as a window, it is desirable to have a smallthickness and a high strength that is not easily broken by an externalimpact since it is required to have a high transmittance and a smallweight. Glass having a high strength can be produced by, for example,chemical strengthening or thermal strengthening. Examples ofstrengthened glass are shown in FIG. 1.

FIG. 1 is a perspective view of glass articles according to variousexemplary embodiments.

Referring to FIG. 1, in an exemplary embodiment, the glass article 100may have the shape of a flat sheet or a flat plate. In another exemplaryembodiment, the glass articles 200 and 300 may have a three-dimensionalshape including bent portions. For example, the edge of the flat portionmay be curved (e.g., the glass article 200) or the entire surface may becurved (e.g., the glass article 300). The shape of the glass articles100, 200 and 300 may be, but is not limited to being, a rectangle whenviewed from the top. For example, the glass articles 100, 200 and 300may have various shapes such as a rounded rectangle, a square, a circle,and an ellipse. In the following description, a glass article having theshape of a rectangular flat plate will be described as an example of theglass article 100. It is, however, to be understood that the presentdisclosure is not limited thereto.

FIG. 2 is a cross-sectional view of a glass article having the shape ofa flat plate according to an exemplary embodiment of the presentdisclosure.

Referring to FIG. 2, the glass article 100 includes a plurality ofsurfaces US, RS and SS. The surface of the glass article may include afirst surface US, a second surface RS and side surfaces SS. In the glassarticle 100 having the shape of a flat plate, the first surface US andthe second surface RS are main surfaces having a large area (e.g., anupper surface and a lower surface), and side surfaces SS are outersurfaces connecting the first surface US with the second surface RS.

The first surface US and the second surface RS are opposed to each otherin the thickness (t) direction. When the glass article 100 serves totransmit light like a window of a display, the light may be mainlyincident on the first surface US or the second surface RS to exitthrough the other.

The thickness t of the glass article 100 is defined as the distancebetween the first surface US and the second surface RS. The thickness tof the glass article 100 may range, but is not limited to, from 0.1 to 2mm. In an exemplary embodiment, the thickness t of the glass article 100may be approximately 0.8 mm or less. In another exemplary embodiment,the thickness t of the glass article 100 may be approximately 0.65 mm orless. In yet another exemplary embodiment, the thickness t of the glassarticle 100 may be approximately 0.55 mm or less. In yet anotherexemplary embodiment, the thickness t of the glass article 100 may beapproximately 0.5 mm or less. In yet another exemplary embodiment, thethickness t of the glass article 100 may be approximately 0.3 mm orless. Although the glass article 100 has the uniform thickness t, it mayhave different thicknesses for different regions.

The strengthened glass article 100 includes compressive regions CSR1 andCSR2 and a tensile region CTR. The compressive regions CSR1 and CSR2refer to regions where compressive stress act, and the tensile regionCTR refer to a region where tensile stress acts. The compressive regionsCSR1 and CSR2 are disposed adjacent to the surfaces US, RS and SS of theglass article 100. The tensile region CTR is disposed in the inside (orcenter) of the glass article 100. The compressive regions may bedisposed adjacent to the side surfaces SS as well as the first surfaceUS and the second surface RS. The depths (compression depths) of thecompressive regions CSR1 and CSR2 extending in the depth direction fromeach of the surfaces US, RS and SS may be, but is not limited to being,substantially uniform. The tensile region CTR may be surrounded by thecompressive regions CSR1 and CSR2.

FIG. 3 is a graph showing the stress profile of a glass articleaccording to an exemplary embodiment of the present disclosure. In thegraph of FIG. 3, the x-axis represents the thickness (t) direction ofthe glass article 100. In FIG. 3, the compressive stress has positivevalues, while the tensile stress has negative values. Herein, themagnitude of the compressive/tensile stress means the absolute valueregardless of its sign.

Referring to FIGS. 2 and 3, the glass article 100 includes a firstcompressive region CSR1 that is extended from the first surface US to afirst depth (a first compression depth DOL1), and a second compressiveregion CSR2 that is extended from the second surface RS to a seconddepth (a second compression depth DOL2). A tensile region CTR isdisposed between the first compression depth DOL1 and the secondcompression depth DOL2. Although not shown in FIG. 3, a compressiveregion and a tensile region may be disposed between opposed sidesurfaces SS of the glass article 100 in a similar manner.

The first compressive region CSR1 and the second compressive region CSR2are resistant to an external impact, thereby suppressing cracks in theglass article 100 or damage to the glass article 100. It can be saidthat the larger the maximum compression stresses CS1 and CS2 of thefirst and second compressive regions CSR1 and CSR2 are, the higher thestrength of the glass article 100 is. Since an external impact isusually transmitted through the surfaces US, RS and SS of the glassarticle 100, it is advantageous to have the maximum compressive stressesCS1 and CS2 at the surfaces US, RS and SS of the glass article 100 interms of durability. The maximum compressive stresses CS1 and CS2 of thefirst and second compressive regions CSR1 and CSR2 may be 700 MPa ormore. For example, the maximum compressive stresses CS1 and CS2 of thefirst and second compressive regions CSR1 and CSR2 may be in the rangeof 800 MPa to 1,050 MPa. In an exemplary embodiment, the maximumcompressive stresses CS1 and CS2 of the first and second compressiveregions CSR1 and CSR2 may be in the range of 850 MPa to 1,000 MPa.

The first compression depth DOL1 and the second compression depth DOL2suppress cracks or grooves formed in the first and second surfaces USand RS from propagating to the tensile region CTR inside the glassarticle 100. The larger the first and second compression depths DOL1 andDOL2 are, the better the propagation of cracks and the like can beprevented.

The first and second compression depths DOL1 and DOL2 may be in therange of 20 μm to 150 μm. In an exemplary embodiment, the first andsecond compression depths DOL1 and DOL2 may be in the range of 50 μm to100 μm. In a particular exemplary embodiment, the first and secondcompression depths DOL1 and DOL2 may range from 70 to 85 μm.

In some exemplary embodiments, the first and second compression depthsDOL1 and DOL2 may satisfy the following relationship with respect to thethickness t of the glass article 100, although not limited thereto:

DOL1, DOL2≥0.1*t   [Mathematical Expression 1]

In the exemplary embodiment of FIG. 3, the compressive stresses of thefirst compressive region CSR1 and the second compressive region CSR2 arethe largest at the surfaces US and RS (see CS1 and CS2), respectively,and decrease toward the inside. Such stress profile may be obtained viaan ion exchange process. The ion exchange process refers to a process ofexchanging ions in the glass article 100 with other ions. By performingthe ion exchange process, the ions at or near the surfaces US, RS, SS ofthe glass article 100 can be replaced or exchanged with larger ionshaving the same valence or oxidation state. For example, when the glassarticle 100 contains a monovalent alkali metal such as Li+, Na+, K+ andRb+, the monovalent cation on the surface may be replaced by Na+, K+,Rb+, or Cs+ ions with larger ionic radius.

FIG. 4 is a schematic diagram illustrating an ion exchange processaccording to an exemplary embodiment of the present disclosure. FIG. 4shows that sodium ions (Na+) inside the glass are exchanged withpotassium ions (K+).

Referring to FIG. 4, when the glass containing sodium ions is exposed topotassium ions by, for example, immersing the glass in a molten saltbath containing potassium nitrate (KNO₃), sodium ions in the glass aredischarged to the outside and the potassium ions can replace them. Theexchanged potassium ions generate compressive stress because they havelarger ionic radius than sodium ions. The more potassium ions areexchanged, the greater the compressive stress becomes. Since the ionexchange takes place through the surface of the glass, the amount ofpotassium ions (i.e., density) on the glass surface is the greatest.Although some of the exchanged potassium ions may diffuse into the glassto increase the compression depth, the amount (density) may be generallyreduced away from the surface. Thus, the glass may have the stressprofile that has the greatest compressive stress on the surface anddecreases toward the inside. However, the exemplary embodiments are notlimited to the above examples. The stress profile may be modifieddepending on the temperature, time and the number of the ion exchangeprocess, whether heat treatment is carried out, etc.

Referring again to FIGS. 2 and 3, the glass article 100 has a neutralstress (the stress value substantially equal to zero) at the firstcompression depth DOL1 and the second compression depth DOL2, and hastensile stress on the inner side. The tensile stress may be constant orincreased toward the center.

The absolute value of the slope of the compressive stress in the stressprofile may be greater than the absolute value of the slope of thetensile stress. On the inner side of the glass article 100, there may bea wide section exhibiting tensile stress and having the average slope ofzero. The width of the section of the tensile region CTR in which theaverage slope is zero (i.e., the thickness of the glass article) may be,but is not limited to being, larger than the first and secondcompression depths DOL1 and DOL2.

The tensile stress in the tensile region CTR may balance the compressionstresses of the first and second compressive regions CSR1 and CSR2. Thatis to say, in the glass article 100, the sum of the compressive stressesmay be equal to the sum of tensile stresses. When the stress profile inthe glass article 100 is represented by the function f(x), the followingrelationship can be established:

∫₀ ^(t) f(x)dx=0   [Mathematical Expression 2]

For the glass article 100 in which the maximum compression stresses CS1and CS2 and the compression depths DOL1 and DOL2 of the firstcompression region CSR1 and the second compression region CSR2 are equalto each other and their profiles approximate a triangular shape, and theprofile of the tensile region CTR generally approximates a rectangularshape, the following relationship may be established:

CT1=(CS1*DOL1)/(t−2*DOL1)   [Mathematical Expression 3]

where CT1 denotes the maximum tensile stress of the tensile region CTR,and CS1 denotes the maximum compressive stress of the first compressiveregion CSR1.

The larger the tensile stress inside the glass article 100 is, the morelikely the fragments to be vigorously released when the glass article100 is broken, and the more likely the glass article 100 is to becrushed from the inside. The maximum tensile stress that meets thefrangibility requirements of the glass article 100 may satisfy thefollowing relationship:

CT1≤−37.6*ln(t)+48.7   [Mathematical Expression 4]

where CT1 is expressed in MPa, thickness t is expressed in mm, and ln(t)denotes the natural logarithm with respect to thickness t.

Although it is desired that the compressive stresses CS1 and CS2 and thecompression depths DOL1 and DOL2 have large values in order to increasethe strength of the glass article 100, the tensile stress is alsoincreased with the sum of the compressive stresses. In order to meet thefrangibility requirements while having a high strength, it is desired toadjust the stress profile such that the maximum compressive stresses CS1and CS2 and the compression depths DOL1 and DOL2 have large values whilethe sum of the compressive stresses (e.g., the area of the compressiveregions in FIG. 3) becomes smaller. The stress profile in the glassarticle 100 can be controlled by an ion exchange process, a heattreatment process, a post-treatment process, or the like. Detaileddescriptions on this will be given later on.

According to some exemplary embodiments of the present disclosure, themaximum tensile stress CT1 of the glass article 100 can satisfy thecondition of Equation 4 above within the range defined by Equation 5below:

−37.6*ln(t)+10≤CT1≤−37.6*ln(t)+48   [Mathematical Expression 5]

According to an exemplary embodiment of the present disclosure, theglass article 100 may have no low-refractive surface layer on the firstsurface US and the second surface RS or may have a low-refractivesurface layer having a very small thickness (e.g., less than 100 nm).This is in contrast to glass having a low-refractive surface layer witha thickness of 100 nm to 500 nm on the surface via an ion exchangeprocess. The glass article 100 according to the exemplary embodiment ofthe present disclosure may be produced by preparing a strengthened glassincluding a low-refractive surface layer and removing all or a part ofthe low-refractive surface layer. Removing the low-refractive surfacelayer may be carried out by etching or polishing. More detaileddescription thereon will be given below with respect to the followingexemplary embodiments.

FIG. 5 is a flowchart for illustrating a method for producing a glassarticle according to an exemplary embodiment of the present disclosure.

Referring to FIG. 5, according to an exemplary embodiment of the presentdisclosure, a method for producing a glass article includes preparing astrengthened glass including a low-refractive surface layer (step S11),and etching away the low-refractive surface layer (step S12).

FIG. 6 is a cross-sectional view of a strengthened glass including alow-refractive surface layer.

Referring to FIG. 6, the strengthened glass 101 may be obtained via anion exchange process. A method for producing the strengthened glass 101will be described in detail later.

The ion exchange process can introduce compressive stress in thevicinity of the surface of the glass while forming a low-refractivesurface layer 130 on the surface. The low-refractive surface layer 130may be formed on all surfaces (the first surface US, the second surfaceRS, and the side surfaces SS) of the strengthened glass 101. Thelow-refractive surface layer 130 is located on the surfaces US, RS andSS of the strengthened glass 101 in the compressive regions CSR1 andCSR2. The low-refractive surface layer 130 has a thickness less than thecompression depth. The thickness of the low-refractive surface layer 130may range from 100 nm to 500 nm. The compressive regions CSR1 and CSR2may have the maximum compressive stress at the surface of thelow-refractive surface layer 130.

The low-refractive surface layer 130 may not be distinguished by nakedeyes from the bulk BLK of the strengthened glass 101 (the portion of thestrengthened glass except the low-refractive surface layer). That is tosay, unlike typical layer structures, the bulk BLK of the strengthenedglass 101 may not be distinguished from the low-refractive surface layer130 by naked eyes. However, the low-refractive surface layer 130 may bedistinguishable from the bulk BLK of the strengthened glass 101 by anoptical method, by compositions thereof, or the like.

The low-refractive surface layer 130 has a lower refractive index thanthe refractive index of the bulk BLK of the strengthened glass 101. Therefractive index of the low-refractive surface layer 130 is larger thanthe refractive index of air and smaller than the refractive index of thebulk BLK of the strengthened glass 101. When the refractive index of thebulk BLK of the strengthened glass 101 is 1.5, the refractive index orthe average refractive index of the low-refractive surface layer 130 maybe equal to 1.48 or less, equal to 1.45 or less, and in some exemplaryembodiments equal to 1.3 or less. The low-refractive surface layer 130may form an optical interface with the bulk BLK of the strengthenedglass.

The low-refractive surface layer 130 may include fine pores or finegrooves therein. The fine pores or fine grooves may be filled with theair to lower the average refractive index of the low-refractive surfacelayer 130.

The low-refractive surface layer 130 may be a silicon rich layer. Thelow-refractive surface layer 130 may have a silicon content higher thanthat of the bulk BLK of the strengthened glass 101. The silicon contentof the low-refractive surface layer 130 is higher because the alkalimetal or alkaline earth metal is removed during a high-temperature salttreatment process for ion exchange of the strengthened glass 101. In anexemplary embodiment, the ratio of the silicon content of thelow-refractive surface layer 130 to the silicon content of the bulk BLKof the strengthened glass 101 may be, but is not limited to, 1.2 to 1.4.

The low-refractive surface layer 130 may be a sodium poor layer. Sincethe low-refractive surface layer 130 is located on the surfaces US, RSand SS of the strengthened glass 101, the sodium may be largelydischarged through the ion exchange, such that the sodium content of thelow-refractive surface layer 130 may be significantly smaller that thesodium content of the bulk BLK of the strengthened glass 101.

The low-refractive surface layer 130 may include a fine crack 135. Thefine crack 135 may be formed by a small friction, impact, or reactionwith moisture in the air during the process of producing glass. The finecrack 135 may be generated during a process of strengthening the glassand may become larger under high temperature conditions of thestrengthening process.

The low-refractive surface layer 130 is located at the outermost side ofthe strengthened glass 101 and may include defects such as the finecrack 135. Such defects on the surface may lower the strength of thestrengthened glass 101.

Accordingly, according to an exemplary embodiment of the presentdisclosure, the method for producing glass article includes etching andremoving the low-refractive surface layer 130 of the strengthened glass101 which is to be processed. By removing the low-refractive surfacelayer 130, it is possible to improve the transmittance of the glassarticle 100 (see FIG. 2), and overcome defects on the surface. As thelow-refractive surface layer 130 is removed, the thickness of the glassarticle 100 (see FIG. 2) may be somewhat reduced.

The etching the low-refractive surface layer 130 may include an acidcleaning process and a base cleaning process. The acid cleaning processmay be carried out using an acidic solution containing at least one of:inorganic acids such as hydrochloric acid, sulfuric acid, nitric acidand hydrogen fluoride; and organic acids such as formic acid, oxalicacid, citric acid, acetic acid, and benzoic acid. The base cleaningprocess may be carried out using a base solution containing at least oneof: hydroxides of alkali metals such as sodium hydroxide, potassiumhydroxide and lithium hydroxide; hydroxides of alkaline earth metal suchas calcium hydroxide; inorganic alkali metal salts such as sodiumcarbonate; organic alkali metal salts such as sodium acetate, andammonia water. FIGS. 7 and 8 illustrate an acid cleaning process usingan acidic solution containing nitric acid, and a base cleaning processusing a basic solution containing sodium hydroxide, respectively.

FIG. 7 is a cross-sectional view showing an acid cleaning process of thestrengthened glass.

Referring to FIG. 7, the strengthened glass 101 including thelow-refractive surface layer 130 is cleaned with an acidic solutioncontaining nitric acid (HNO₃). Although the acid cleaning process may becarried out by immersing the strengthened glass 101 into an acidicsolution to stir it, it may also be carried out by spraying or othermethods.

The content of nitric acid in the acid solution may be, e.g.,approximately 6 wt % or less, e.g., approximately 2 wt % to 5 wt %. ThepH of the acidic solution may range from 1 to 3. The acid cleaningprocess may be carried out at a temperature of 30° C. to 50° C. for 0.5to 5 minutes or 2 to 4 minutes.

Even after the above-described acid cleaning process has been completed,the low-refractive surface layer 130 is hardly removed.

FIG. 8 is a cross-sectional view showing a base cleaning process ofstrengthened glass.

After the acid cleaning process, the strengthened glass 101 is cleanedwith a basic solution containing sodium hydroxide (NaOH). Although thebase cleaning process may be carried out by immersing the strengthenedglass 101 into a base solution to stir it, it may also be carried out byspraying or other methods.

The content of sodium hydroxide in the base solution may be, e.g.,approximately 6 wt % or less, e.g., approximately 2 wt % to 5 wt %. ThepH of the base solution may range from 12 to 14. The base cleaningprocess may be carried out at a temperature of 30° C. to 50° C. for 0.5to 5 minutes or 2 to 4 minutes.

After the base cleaning process has been completed, the surface of thestrengthened glass 101 may be etched to a depth of approximately 500 nm,such that the low-refractive surface layer 130 may be removed. The finecrack 135 formed on the surface can also be removed together with thelow-refractive surface layer 130. After the low-refractive surface layer130 has been completely removed, a glass article as shown in FIG. 2 canbe obtained. The glass article 100 of FIG. 2 may have no opticalinterface throughout the glass article 100 when compared to thestrengthened glass 101 of FIG. 6. In addition, defects on the surfaceare overcome and the strength is improved as the low-refractive surfacelayer 130 has been removed.

In some implementations, a part of the low-refractive surface layer 130may remain, in which case the thickness of the low-refractive surfacelayer 130 is still reduced. Since the fine crack 135 penetrate in thedepth direction from the surface, such a fine crack may be removed orreduced if the low-refractive surface layer 130 is removed in the depthdirection from the surface by the etching process. Accordingly, even ifthe low-refractive surface layer 130 remains at a small thickness ofless than 100 nm, the strength of the glass article 100 can be enhancedby the etching process.

As described above, the low-refractive surface layer 130 is removedmainly by the base cleaning process. However, when the base cleaningprocess alone is performed without the acid cleaning process, thelow-refractive surface layer 130 is not efficiently etched away. It isbelieved that the acid cleaning process itself does not remove thelow-refractive surface layer 130 but serves to transform thelow-refractive surface layer 130 into a state that can be easily removedby the base cleaning process. For the same reason, it is preferable tocarry out the acid cleaning process first and then the basic cleaningprocess.

The etching process including the acid cleaning and base cleaningprocesses mainly etches the low-refractive surface layer 130. In anexemplary embodiment, the etch rate of the bulk BLK of the strengthenedglass 101 with respect to the cleaning solution is lower than the etchrate of the low-refractive surface layer 130. After the low-refractivesurface layer 130 has been completely removed, the surface of the bulkBLK of the strengthened glass may be exposed to the cleaning solutiondepending on the concentration of the cleaning solution, time,temperature, etc. However, if the etch rate of the bulk BLK of thestrengthened glass is low, it is possible to prevent the bulk BLK frombeing overly etched.

When the surface of the strengthened glass 101 is etched, the stressprofile inside it may change as well.

FIG. 9 is a graph showing the stress profile before and after theetching process. For convenience of illustration, the stress profile issimplified in FIG. 9.

Referring to FIG. 9, the strengthened glass has the maximum compressivestress CS1′ at the surface before the etching. The maximum compressivestress also changes as the surface of the strengthened glass is removedby the etching process. As mentioned earlier, the compressive stress isproportional to the density of the exchanged ions. During the etchingprocess, no ion is supplied newly, and the heat energy for diffusingions is not so large. Thus, the position of the exchanged ions hardlychanges. Therefore, the compressive stress at the surface can bemaintained as the original profile, except the portion removed by theetching. If the original profile had a shape that decreased withthickness (depth), it would have the maximum compressive stress CS atthe surface of the glass article even after the etching.

Since the surface of the strengthened glass having the maximumcompressive stress CS1′ before the etching is removed by the etching,the maximum compressive stress CS1 after the etching is lower than themaximum compressive stress CS1′ before the etching. Although thethickness removed by the etching is merely a very small fraction of thecompression depth, the reduction rate of the maximum compressive stressmay vary depending on the slope of the compressive stress profile. Ifthe slope of the compressive stress profile CS1′ near the surface issteep, the maximum compressive stress CS1 may be significantly reducedeven if a small thickness of the surface has been etched away.

In an exemplary embodiment, the reduction rate of the maximumcompressive stress by the etching ((CS1′−CS1)/CS1′) may be equal to orless than 10%. The reduction in the maximum compressive stress(CS1′−CS1) by the etching may range from 10 MPa to 100 MPa. For example,the reduction (CS1′−CS1) may be in the range of 50 MPa to 100 MPa, or inthe range of approximately 60 MPa to 70 MPa.

As the maximum compressive stress CS1 decreases after the etching, thesum of the compressive stresses of the compressive regions can also bereduced. Accordingly, the sum of the tensile stresses of the tensileregions can also be reduced to satisfy Equation 2 above. As the sum oftensile stresses changes, the stress profile in the tensile regionchanges, and the neutral stress point also changes. As can be seen fromresults of the experiments, the first compression depth DOL1′ before theetching is substantially the same as the first compression depth DOL1after the etching, without any significant change. This means that theneutral stress point has moved to the inside of the strengthened glass.

The maximum tensile stress can also be reduced as the sum of the tensilestresses is reduced by the etching. The present disclosure, however, isnot limited thereto. The maximum tensile stress may be maintained if itis possible to compensate for the reduced stress only by a change in theneutral stress point and/or a change in the slope of the tensile stressprofile.

In an exemplary embodiment where the maximum tensile stress is reducedby the etching, the frangibility requirements of the glass article ofEquation 4 above is applied to the glass article after the etching. Thestrengthened glass before the etching, which is not a complete articleyet, may not necessary have to meet the requirements. In view of theabove, according to some exemplary embodiments of the presentdisclosure, the strengthened glass before the etching and the glassarticle after the etching may satisfy the following relationship:

CT1′>−37.6*ln(t′)+48.7, CT1≤−37.6*ln(t)+48.7   [Mathematical Expression6]

where CT1′ denotes the maximum tensile stress before the etching, t′denotes the thickness of the strengthened glass, CT1 denotes the maximumtensile stress after the etching, and t denotes the thickness of theglass article.

According to Equation 6 above, the strengthened glass can be producedunder conditions exceeding the frangibility requirements according toEquation 4, and the tensile stresses can be adjusted to satisfy Equation4 by the etching. The present disclosure, however, is not limitedthereto. The maximum tensile stresses CT1′ and CT1 of the strengthenedglass before as well as after the etching may satisfy Equation 4.

FIGS. 10 and 11 are images when viewed from the top for comparing thesurfaces of the glass articles according to the etching techniques. FIG.10 is an image of a glass article obtained by performing an etchingprocess on a strengthened glass with hydrofluoric acid (HF), taken by ascanning electron microscope (SEM). FIG. 11 is an image of a glassarticle obtained by performing an etching process on the strengthenedglass according to the exemplary embodiments of FIGS. 7 and 11, taken bya scanning electron microscope (SEM).

Hydrofluoric acid (HF) can also be effectively used to remove finecracks on the surface of the strengthened glass. For example, a finecrack can be removed by processing the surface of the strengthened glasswith hydrofluoric acid (HF), and polishing or etching away the peripheryof the fine crack so that a gentle recess is formed around the finecrack. However, since the etching with hydrofluoric acid (HF) leavesrecesses in the shape of rice grains on the surface of the glass articleas shown in FIG. 10, the optical characteristics are degraded and therecesses can be seen by a user.

In contrast, if the etching is carried out with an acid solutioncontaining nitric acid and a basic solution containing sodium hydroxide,fine cracks can be removed without leaving recesses in the shape of ricegrains on the surface of the glass article, to provide a substantiallyflat surface as shown in FIG. 11.

Hereinafter, a method for producing strengthened glass will bedescribed, which is prepared before the etching process of thelow-refractive surface layer.

Initially, the glass is prepared, which is not strengthened yet. Theglass before it is strengthened may be produced by various techniquessuch as a floating method, a fusion method, and a slot down-draw method.The glass before it is strengthened may be an alkali aluminosilicateglass or the like. The compositions of the glass before it isstrengthened may include SiO₂ and Al₂O₃. The compositions of the glassbefore it is strengthened may further include Na₂O. The compositions ofthe glass before it is strengthened may include at least one of K₂O,B₂O₃, Li₂O, MgO, CaO, ZnO, ZrO₂, Fe₂O₃, SnO₂ and P₂O₅. In some exemplaryembodiments, the glass before it is strengthened may containsubstantially no Li₂O and P₂O₅. In some exemplary embodiments, the glassbefore it is strengthened may contain substantially no Li₂O, P₂O₅ andB₂O₃. According to the exemplary embodiment, even if the glass articlecontains no Li₂O, P₂O₅ and/or B₂O₃, it can have a compressive stress of700 MPa or higher via the strengthening process and the process ofremoving the low-refractive surface layer and can have a sufficientdepth for satisfying Equation 1 above.

Subsequently, the glass is strengthened. The glass may be strengthenedby a first ion exchange process and a second ion exchange process.

The first ion exchange process may include exposing the glass to amolten salt containing potassium ions (K+). The molten salt may be, forexample, a salt mixed with sodium nitrate (NaNO₃) and potassium nitrate(KNO₃). The first ion exchange process may be carried out at atemperature of ±20° C., which is lower than the strain point by 50° C.For example, if the strain point of the glass is approximately 580° C.,the first ion exchange process may be carried out at a temperature ofapproximately 500° C. or higher (e.g., 530° C.). The time for which thefirst ion exchange process is carried out may range, but is not limitedto, 3 hours to 8 hours (e.g., approximately 5 hours). Through the firstion exchange process, potassium ions may enter the glass to introducecompressive stress near the surface of the glass.

After the first ion exchange process, a stress-relieving process may befurther carried out. The stress-relieving process may be carried out ata temperature of approximately 500° C. or higher (e.g., approximately530° C.) for 1 to 3 hours (e.g., approximately 2 hours). Via thestress-relieving process, the maximum compressive stress may be reduced,and the potassium ions diffuse into the inside and the compression depthmay be increased. The stress-relieving process may be carried out in airor in liquid. The stress-relieving process may be carried out in aliquid by immersing it into in a salt mixture of potassium ions andsodium ions to perform heat treatment. The stress-relieving process maybe eliminated.

After the stress-relieving process, the second ion exchange process iscarried out. The second ion exchange process may be carried out byexposing the glass to a molten single salt containing potassium ions.The second ion exchange process may be carried out at a lowertemperature and for a shorter time than the first ion exchange process.For example, the second ion exchange process may be carried out at atemperature from 380° C. to 460° C. (e.g., approximately 420° C.) for 1to 3 hours or 1.3 to 2 hours.

After the second ion exchange process is carried out, the compressivestress can be increased greatly at a shallow position of the surface ofthe glass.

After the first ion exchange process, the stress-relieving process andsecond ion exchange process, the glass article may have a high surfacecompressive stresses and a sufficient compression depth. The compressivestress profile may have a steep slope near the surface of the glassarticle, and may have a slope becoming gentler toward the inside of theglass article. For example, the compressive stress profile may include afirst section having the average slope (absolute value) of 40 MPa/μm to200 MPa/μm from the surface to a first point, and a second section moredistant from the surface than the first section and having the averageslope (absolute value) equal to or less than 2 MPa/μm. Both the firstsection and the second section may lie within the compressive region.The first section may be extended from the surface to a depth exceeding15 μm. In some exemplary embodiments, the compressive stress profile mayinclude a point in the compressive region at which the average slope is0 MPa/μm.

FIGS. 12 and 13 are graphs illustrating the stress profiles of the glassafter being subjected to the second ion exchange process according tovarious exemplary embodiments.

In the graph shown in FIG. 12, a first curve PL11 represents the stressprofile of the glass that was subjected to the first ion exchangeprocess and then the second ion exchange process for approximately 30minutes. A second curve PL12 represents the stress profile of the glassthat was subjected to the first ion exchange process and then the secondion exchange process for approximately 60 minutes. A third curve PL13represents the stress profile of the glass that was subjected to thefirst ion exchange process and then the second ion exchange process forapproximately 90 minutes. A fourth curve PL14 represents the stressprofile of the glass that was subjected to the first ion exchangeprocess and then the second ion exchange process for approximately 120minutes. Referring to FIG. 12, it can be seen that the transitionpoints, at which the slopes change sharply, vary depending on the periodof time in which the second ion exchange process is carried out. In thegraph shown in FIG. 12, the transition points of the first to fourthcurves PL11 to PL14 are 9 μm, 12 μm, 15 μm and 17 μm, respectively.Accordingly, the depths of the transition points tend to increase as theperiod of time of the second ion exchange process increases.

In the graph shown in FIG. 13, a fifth curve PL21 represents the stressprofile of the glass that was subjected to the first ion exchangeprocess, a stress-relieving process and then the second strengtheningprocess for approximately 30 minutes. A sixth curve PL22 represents thestress profile of the glass that was subjected to the first ion exchangeprocess, the stress-relieving process and then the second strengtheningprocess for approximately 120 minutes. A seventh curve PL23 representsthe stress profile of the glass that was subjected to the first ionexchange process, and then the second strengthening process forapproximately 60 minutes, without the stress-relieving process. Aneighth curve PL24 represents the stress profile of the glass that wassubjected to the first ion exchange process, and then the secondstrengthening process for approximately 90 minutes, without thestress-relieving process.

Referring to FIG. 13, it can be seen that the transition points can becontrolled also by adding the stress-relieving process. Specifically, ascan be seen from the fifth curve PL21 and the sixth curve PL22, even ifthe stress-relieving process is further carried out, the positions ofthe transition points can be changed by controlling the duration of thesecond strengthening process. Further, as indicated by the seventh curvePL23 and the eighth curve PL24, the positions of the transition pointscan be easily controlled by controlling the period of time the secondstrengthening step even if the stress-relieving process is not furtherperformed.

A more detailed description and various embodiments of theabove-mentioned method for producing strengthened glass are disclosed inKorean Patent Application No. 10-2017-0048080 filed by the applicant onApr. 13, 2017, the entirety of which is incorporated herein byreference.

FIG. 14 is a flowchart for illustrating a method for producing a glassarticle according to another exemplary embodiment of the presentdisclosure.

Referring to FIG. 14, according to the exemplary embodiment of thepresent disclosure, a method for producing a glass article includespreparing a strengthened glass including a low-refractive surface layer(step S21), and polishing the low-refractive surface layer (step S22).

The preparing the strengthened glass including the low-refractivesurface layer (step S21) is identical to that of the exemplaryembodiments of FIGS. 5 and 6; and, therefore, the redundant descriptionwill be omitted.

Subsequently, the low-refractive surface layer is polished (step S22).

FIG. 15 is a cross-sectional view showing a polishing process of thestrengthened glass.

Referring to FIG. 15, the polishing process may be carried out by achemical mechanical polishing method. Specifically, the first surface USand the second surface RS of the strengthened glass 130 to be processedare polished using a chemical mechanical polishing apparatus 510 and apolishing slurry 520. Although it is shown that the first surface US andthe second surface RS are polished simultaneously, the first surface USand the second surface RS may be polished sequentially as well. Forexample, the strengthened glass 101 may be placed such that the secondsurface RS faces the stage (not shown) of the chemical mechanicalpolishing apparatus 510, the first surface US exposed upward ispolished, and then the strengthened glass 101 is turned over such thatthe second surface RS is polished. According to another exemplaryembodiment of the present disclosure, the first surface US or the secondsurface RS may be subjected to the polishing process.

The polishing thickness may be adjusted, for example, in the range of100 nm to 1,000 nm (e.g., approximately 500 nm). The first and secondsurfaces may be polished to the same depth or different depths.

FIG. 16 is an image showing a cross section of the glass article afterthe polishing process has been completed.

Referring to FIG. 16, the first surface US and the second surface RS ofthe glass article 102 are removed by polishing the low-refractivesurface layer, but the side surfaces SS are not removed in doing so,such that the low-refractive surface layer 131 may remain on the sidesurfaces SS. Specifically, a first surface US and a second surface RS ofa glass article 102 are identical to those of the glass article shown inFIG. 2. However, side surfaces SS are different in that the sidesurfaces SS include the low-refractive surface layer 131. By removingthe low-refractive surface layer from the first surface US and thesecond surface RS, no optical interface is formed in the thicknessdirection section from the first surface US to the second surface RS, sothat the refractive index can be kept constant. The thickness of thelow-refractive surface layer 131 remaining on the side surfaces SS mayrange from 100 nm to 500 nm. Even after the polishing process, someportion of the low-refractive surface layer may remain on the firstsurface US or the second surface RS. The thickness of the low-refractivesurface layer remaining on the first surface US or the second surface RSmay be smaller than the thickness of the low-refractive surface layer131 remaining on the side surfaces SS.

As the low-refractive surface layer 131 remains on the side surfaces SSof the glass article 102, there may be fine cracks or variations inoptical characteristics. However, unlike the first surface US or thesecond surface RS, the side surfaces SS of the glass article 102 do notcontribute greatly to light transmission. Therefore, the fine cracks onthe side surfaces SS have little effect on the overall strength of theglass article 102. Therefore, the glass article 102 of FIG. 16 can alsohave the optical characteristics and strength comparable to those of theexemplary embodiment of FIG. 2.

The polishing process according to the exemplary embodiment of thepresent disclosure can also be utilized to achieve the uniformcompressive stress characteristics at the first surface and the secondsurface of the glass article.

For example, the floating method, which is one of the techniques forproducing a glass plate, is carried out by flowing glass compositionsinto a tin bath. In doing so, the surface of the glass plate in contactwith the bath may have different compositions from the surface not incontact with the bath. As a result, after the process of strengtheningthe glass, there may be deviations in the compressive stress ofapproximately 40 MPa between the surface in contact with the tin bath(e.g., the first surface) and the surface not in contact with the tinbath (e.g., the second surface). In this regard, by removing the surfaceof the glass to an appropriate thickness by polishing, it is possible toreduce the deviations in the compressive stresses between the surfaces.For example, it is possible to reduce the deviations in the compressivestresses to 20 MPa or less, or 10 MPa or less. In the polishing process,it is possible to select whether or not to polish each surface, and thepolishing thickness of the first surface and the second surface can beadjusted differently. Thus, the compressive stress of the surface incontact with the tin bath and the surface not in contact with the tinbath can be controlled individually. For example, by polishing more thesurface with relatively high compressive stress, it is possible to moreeasily control the deviations in the compressive stress between thesurfaces.

The polishing of the surface of the glass article may be used incombination with the etching. For example, if the compressive stress atthe first surface is greater or is expected to be greater than thecompressive stress at the second surface after the etching of the glassarticle, the first surface may be polished to reduce the compressivestress at the first surface so that it becomes substantially equal tothe compressive stress at the second surface. On the contrary, prior tothe etching the glass article, the polishing process may be carried outto reduce deviations in the compressive stress between the surfaces.

On the other hand, if the glass article 102 is polished using thepolishing slurry 520, there may be a certain surface roughness dependingon the particle size of the polishing slurry 520.

To find out the effects of the polishing process on the surfaceroughness of the glass article 102, two glass article samples that hadbeen subjected to the polishing process were prepared.

FIG. 17 is an image of glass article sample 1 when viewed from the top.FIG. 18 is an image of glass article sample 2 when viewed from the top.

It can be seen from FIGS. 17 and 18 that there are fine patterns on thesurfaces of both of the samples after the polishing process.

To meausre the surface roughness, the value of surface roughness Ra andthe values of surface roughness Rz of glass article samples 1 and 2 wereobtained. The glass article before the polishing had the value of Ra of0.444 nm and the value of Rz of 7.1297 nm. Glass article sample 1 afterthe polishing had the value of Ra of 1.633 nm and the value of Rz of44.286 nm. Glass article sample 2 after the polishing had the value ofRa of 1.404 nm and the value of Rz of 22.928 nm. Although the roughnesswas slightly increased by the polishing, it still lies within the rangeof 0.5 nm to 50 nm, which allows for the transmittance. Thus, it can beseen that the increase in the roughness due to the polishing hardlyaffects the optical characteristics of the glass article.

To evaluate the effects of the etching processes on the strength of theglass articles according to the exemplary embodiments, iron ball droptests were conducted on several glass article samples. The iron balldrop tests were carried out by dropping approximately 150 g of an iron(Fe) ball onto the glass article samples to measure the drop height tobreak the glass article samples. The higher the drop height is, thehigher the strength of the glass article sample is, which means that thesample can withstand stronger impact.

FIGS. 19 to 21 are graphs showing the results of the ball drop tests onthe glass articles according to various exemplary embodiments.

The glass article samples of FIG. 19 had been subjected to a first ionexchange process in a neutral salt consisting of 70 wt % potassiumnitrate (KNO₃) and approximately 30 wt % sodium nitrate (NaNO₃) at atemperature of approximately 530° C., and then a second ion exchangeprocess in a neutral salt consisting of approximately 100 wt % potassiumnitrate (KNO₃) at a temperature of approximately 420° C.

The glass article samples of FIG. 20 had been subjected to a first ionexchange process in a neutral salt consisting of 70 wt % potassiumnitrate (KNO₃) and approximately 30 wt % sodium nitrate (NaNO₃) at atemperature of approximately 530° C., heat treatment at a temperature ofapproximately 530° C., and again a second ion exchange process in aneutral salt consisting of approximately 100 wt % potassium nitrate(KNO₃) at a temperature of approximately 420° C.

Similarly to the samples of FIG. 20, the glass article samples of FIG.21 had been subjected to a first ion exchange process in a neutral saltconsisting of 70 wt % potassium nitrate (KNO₃) and approximately 30 wt %sodium nitrate (NaNO₃) at a temperature of approximately 530° C., heattreatment at a temperature of approximately 530° C., and again a secondion exchange process in a neutral salt consisting of approximately 100wt % potassium nitrate (KNO₃) at a temperature of approximately 420° C.It is, however, to be noted that the heat treatment was carried out fortwo times longer than the glass article samples of FIG. 20.

All of the tests of FIGS. 19 to 21 were performed on 14 glass articlesamples before and after the etching process. In the etching process,the acid cleaning process was carried out first and then the basecleaning process was carried out.

Referring to FIG. 19, breakage heights of the glass article samplesbefore the etching were relatively wide, ranging from approximately 35cm to about 95 cm. Among them, as can be seen from the valid data (databelonging to the boxes of the drawing), the breakage heights of thesamples before the etching were approximately 45 cm to 67 cm. Incontrast, the breakage heights of the samples after the etching wereuniform, i.e., 110 cm.

Referring to FIG. 20, breakage heights of the glass article samplesbefore the etching were relatively wide, ranging from approximately 20cm to about 90 cm. Among them, as can be seen from the valid data (databelonging to the boxes of the drawing), the breakage heights of thesamples before the etching were approximately 35 cm to 60 cm. Thebreakage heights of the samples after the etching were approximately 100to 110 cm.

Referring to FIG. 21, the breakage heights of the glass article samplesbefore the etching were approximately 70 cm to 90 cm, with respect tothe valid data (data belonging to the boxes of the drawing). Inaddition, the breakage heights of the samples after the etching wereuniform, i.e., 110 cm.

It can be seen that the breakage heights of all the samples of FIGS. 19to 21 were improved and became uniform after the etching.

In order to evaluate how the compressive stress and compressive depthvary with the etching process (the acid cleaning process and then thebase cleaning process), compressive stress and compression depth beforeand after the etching were measured for 21 glass article samples. Theresults are shown in Table 1 and in the graphs of FIGS. 22 and 23.

TABLE 1 Before Etching After Etching CS(MPa) DOL(μm) CS(MPa) DOL(μm)982.8 75.4 904.5 75.2 950.1 82.5 891.6 77 948.2 81.5 899.8 77.4 963.876.6 834.5 81.5 947.4 78.2 858.9 79.4 930 78.2 853.9 78.2 955.9 78.9842.7 78.9 968.3 78.3 879.8 77.6 958.4 78.6 893.9 78.6 959.7 76.5 874.281.1 947.1 79 875.5 80.6 937.5 79.2 862.3 78.8 949.1 82 850.7 80.4 964.780.5 844.7 80.9 937.7 80.9 867.3 81.4 955.9 80.9 895.2 80.1 943.2 81.3885.1 80.4 944.8 78 853.5 80 926.4 81.6 895.5 76.7 944 80.4 865.8 80.3950.7 79.4 871.5 79.2

FIG. 22 is a graph showing the compressive stresses of glass articlesamples before and after the etching process. FIG. 23 is a graph showingthe compression depth of the glass article samples before and after theetching process.

Referring to Table 1 and FIG. 22, the average of the compressivestresses of the glass article samples before the etching wasapproximately 957 MPa, while the average of the compressive stresses ofthe glass article samples after the etching was approximately 890 MPa,which was reduced by 60 MPa to 70 MPa in general. The distribution ofthe compressive stresses became narrower after the etching.

Referring to Table 1 and FIG. 23, the average of the compression depthsof the glass article samples before the etching was approximately 78.3μm, while the average of the compression depths of the glass articlesamples after the etching was approximately 78.2 μm. Considering themeasurement errors, it is interpreted that there is no significantdifference in the compression depth before and after the etching.

Next, in order to evaluate how the compressive stress and compressivedepth vary with the polishing process, the compression stress andcompression depth before and after the etching were measured for 38glass article samples. The results are shown in Table 2 and in thegraphs of FIGS. 24 and 25.

TABLE 2 Before Polishing After Polishing CS(MPa) DOL(μm) CS(MPa) DOL(μm)907 75 874 75.6 935 74 893 73.7 902 74 880 75.7 946 72 896 73.5 912 75878 75.4 940 73 906 73.4 916 75 878 75.7 945 72 906 73.2 910 75.3 87775.8 935 72.4 898 73.6 910 76.2 873 75.6 947 73.2 906 72.7 918 75.6 88072.7 942 73.8 903 73.5 908 75.8 895 71.7 934 73.4 877 74.5 904 73.8 89174.9 940 73.4 895 73.7 907 75.4 898 72.7 930 71.4 888 73.6 905 75.4 89074.6 933 74.6 904 73.4 910 74.8 900 72.7 938 71.9 892 74.3 902 75.1 89777 927 72.8 915 73.1 896 76.1 933 72.3 932 71.7 920 75 924 73.7 898 72.7895 75.3 888 73.6 897 75.2 890 74.6 928 73 904 73.4 900 75.1 900 72.7928 74 892 74.3 932 72.9 897 77 902 75.1 915 73.1 936 72.7 933 72.3 90375.7 920 75

FIG. 24 is a graph showing the compressive stresses of glass articlesamples before and after the polishing process. FIG. 25 is a graphshowing the compressive depth of glass article samples before and afterthe polishing process.

Referring to Table 2 and FIG. 24, the average of the compressivestresses of the glass article samples before the polishing wasapproximately 928 MPa, while the average of the compressive stresses ofthe glass article samples after the polishing was approximately 897 MPa,which was reduced by 10 MPa to 50 MPa in general. The distribution ofthe compressive stresses became narrower after the polishing.

Referring to Table 2 and FIG. 25, the average of the compression depthsof the glass article samples before the polishing was approximately 74μm, and the average of the compression depths of the glass articlesamples after the polishing was approximately 74 μm, which is almostequal to the average before the polishing.

While exemplary embodiments are described above, it is not intended thatthese embodiments describe all possible forms of the inventive conceptof the present disclosure. Rather, the words used in the specificationare words of description rather than limitation, and it is understoodthat various changes may be made without departing from the spirit andscope of the inventive concept of the present disclosure. Additionally,the features of various implementing embodiments may be combined to formfurther exemplary embodiments of the present disclosure.

What is claimed is:
 1. A method for producing a glass article, themethod comprising: preparing a glass to be processed, the glasscomprising a glass bulk and a low-refractive surface layer disposed onthe glass bulk; and etching away the low-refractive surface layer toform an etched glass, wherein the etching away the low-refractivesurface layer comprises: cleaning the low-refractive surface layer withan acid solution; and cleaning the low-refractive surface layer with abase solution after the cleaning it with the acid solution.
 2. Themethod of claim 1, wherein the cleaning with the acid solution iscarried out for 0.5 to 5 minutes using an acidic solution containingnitric acid in an amount between 2 wt % and 5 wt %, and wherein thecleaning with the base solution is carried out for 0.5 to 5 minutesusing a base solution containing sodium hydroxide in an amount between 2wt % and 5 wt %.
 3. The method of claim 2, wherein the low-refractivesurface layer is substantially not removed during the cleaning with theacid solution, and is substantially removed during the cleaning with thebase solution.
 4. The method of claim 1, wherein a thickness of thelow-refractive surface layer of the glass ranges from 100 to 500 nm. 5.The method of claim 4, wherein the etching away the low-refractivesurface layer to form an etched glass comprises removing thelow-refractive surface layer completely.
 6. The method of claim 1,wherein the thickness of the low-refractive surface layer is less than100 nm after the etching away the low-refractive surface layer to forman etched glass.
 7. The method of claim 1, wherein a refractive index ofthe low-refractive surface layer is smaller than a refractive index ofthe glass bulk and is greater than a refractive index of air.
 8. Themethod of claim 1, wherein the glass comprises a compressive regiondisposed adjacent to a surface thereof and a tensile region disposedinside it, wherein the low-refractive surface layer is disposed in thecompressive region, and wherein a thickness of the low-refractivesurface layer is smaller than a compression depth of the compressiveregion.
 9. The method of claim 8, wherein a maximum compressive stressof the etched glass is less than a maximum compressive stress of theglass.
 10. The method of claim 9, wherein a difference between themaximum compressive stress of the glass and the maximum compressivestress of the etched glass ranges from 10 MPa to 100 MPa.
 11. The methodof claim 8, wherein a compression depth of the etched glass issubstantially equal to the compression depth of the glass.
 12. Themethod of claim 8, wherein a maximum tensile stress (CT1) of the etchedglass satisfiesCT1≤−37.6*ln(t)+48.7 where CT1 is expressed in MPa, and t denotes thethickness of the etched glass in mm.
 13. The method of claim 12, whereina maximum tensile stress of the glass satisfiesCT1′>−37.6*ln(t′)+48.7 where CT1′ is expressed in MPa, and t′ denotesthe thickness of the glass in mm.
 14. The method of claim 8, wherein thecompressive region has the maximum compressive stress at a surface ofthe low-refractive surface layer.
 15. A method for producing a glassarticle, the method comprising: preparing a glass to be processed, theglass comprising a glass bulk and a low-refractive surface layerdisposed on the glass bulk; and polishing a surface of the glass to forma polished glass, wherein the polishing the surface of the glasscomprises: removing at least partially the low-refractive surface layer.16. The method of claim 15, wherein the glass comprises a first surfaceand a second surface opposed to the first surface, and wherein thepolishing the surface of the glass comprises: polishing the firstsurface, and polishing the second surface.
 17. The method of claim 15,wherein a thickness of the polished surface ranges from 100 nm to 1,000nm.
 18. The method of claim 17, wherein a roughness of the surface ofthe polished glass ranges from 0.5 nm to 50 nm.
 19. The method of claim15, wherein a thickness of the low-refractive surface layer of the glassranges from 100 nm to 500 nm.
 20. The method of claim 15, wherein arefractive index of the low-refractive surface layer is smaller than arefractive index of the glass bulk and is greater than a refractiveindex of air.
 21. The method of claim 15, wherein the glass comprises acompressive region disposed adjacent to a surface thereof and a tensileregion disposed inside it, wherein the low-refractive surface layer isdisposed in the compressive region, and wherein a thickness of thelow-refractive surface layer is smaller than a compression depth of thecompressive region.
 22. The method of claim 21, wherein a maximumcompressive stress of the polished glass is less than a maximumcompressive stress of the glass.
 23. The method of claim 22, wherein adifference between the maximum compressive stress of the glass and themaximum compressive stress of the polished glass ranges from 10 MPa to100 MPa.
 24. The method of claim 21, wherein a compression depth of thepolished glass is substantially equal to the compression depth of theglass.
 25. The method of claim 21, wherein a maximum tensile stress(CT1) of the polished glass satisfiesCT1≤−37.6*ln(t)+48.7 where CT1 is expressed in MPa, and t denotes thethickness of the polished glass in mm.
 26. The method of claim 25,wherein a maximum tensile stress of the glass satisfiesCT1′>−37.6*ln(t′)+48.7 where CT1′ is expressed in MPa, and t′ denotesthe thickness of the glass in mm.
 27. The method of claim 21, whereinthe compressive region has the maximum compressive stress at a surfaceof the low-refractive surface layer.
 28. A method for producing a glassarticle, the method comprising: preparing a glass to be processed thathas a first surface and a second surface opposed to the first surface,wherein the glass has a first maximum compressive stress at the firstsurface and a second maximum compressive stress at the second surface;and polishing the first surface and/or the second surface to reducedeviations between the first maximum compressive stress and the secondmaximum compressive stress.
 29. The method of claim 28, wherein theglass is produced by a floating technique, and wherein the first surfaceof the glass comes in contact with a floating fluid, and the secondsurface of the glass does not come in contact with the floating fluid.30. A glass article comprising a glass comprising glass bulk and alow-refractive surface layer disposed on the glass bulk, the glasscomprising a compressive region disposed adjacent to a surface of theglass and a tensile region disposed inside the glass, wherein arefractive index of the low-refractive surface layer is smaller than arefractive index of the glass bulk and is greater than a refractiveindex of air, wherein the low-refractive surface layer is disposedwithin the compressive region, and wherein a thickness of thelow-refractive surface layer is less than 100 nm and is smaller than acompression depth of the compressive region.
 31. The glass article ofclaim 30, wherein the compressive region has the maximum compressivestress at a surface of the low-refractive surface layer.
 32. A glassarticle comprising a glass comprising a first surface, a second surfaceopposed to the first surface and side surfaces, wherein the glasscomprises a first compressive region having a first compression depthfrom the first surface, a second compressive region having a secondcompression depth from the second surface, and a tensile region disposedbetween the first compressive region and the second compressive region,wherein the glass comprises glass bulk and a side low-refractive surfacelayer disposed on side surfaces of the glass bulk, and wherein arefractive index of the side low-refractive surface layer is smallerthan a refractive index of the glass bulk and greater than a refractiveindex of air.
 33. The glass article of claim 32, wherein the glass hassubstantially same refractive index from the first surface to the secondsurface.
 34. The glass article of claim 32, wherein a thickness of theside low-refractive surface layer ranges from 100 nm to 500 nm.
 35. Theglass article of claim 34, further comprising: a first low-refractivesurface layer disposed on the first surface of the glass bulk, whereinthe first low-refractive surface layer is made of a same material as theside low-refractive surface layer, and wherein a thickness of the firstlow-refractive surface layer is smaller than a thickness of the sidelow-refractive surface layer.
 36. The glass article of claim 32, whereina roughness of each of the first surface and the second surface rangesfrom 0.5 nm to 50 nm.